The technique of pulse compression involves the rearrangement of the temporal distribution of energy in a pulse in such a way that a long pulse with a given energy is transformed to a shorter pulse with the same energy. The instantaneous power during the shortened pulse is therefore greater than the instantaneous power during the long pulse, since the total energy is the same for both. Pulse compression is useful in target sensor systems for several reasons. Because target tracking is essentially a process of measurement of the time of arrival of a waveform, pulse compression allows a more precise measurement of arrival time and therefore of the range to the target. If the active sensor is peak power limited, as is usually the case, pulse compression allows long-pulse, limited peak power systems to have performance equivalent to a shorter-pulse higher-peak power system.
Ideally, pulse compression is implemented with matched filters where the processing device is a network with impulse response matched to the time reverse of the long-pulse waveform. This matched-filter operation results in maximizing the signal-to-noise ratio and in optimum detection of the target. FIG. 1 illustrates the concept of pulse compression.
Pulse-compression techniques can be implemented using trasveral filtering. Essentially, the method is to delay the energy arriving early in the long pulse period, add it coherently with the energy that arrives later in the pulse, an output the resulting shorter pulse. Two technical issues that arise in the evaluation of the effectiveness of the pulse-compression systems are sidelobes in the compressed-pulse waveform and the response of the transversed filter to other, nonmatched waveforms which may be present in the received signal. The nonmatched waveforms could be the result of receiving the transmission of other deployed sensors or the result of the transmissions of intentional jammers. FIG. 2 illustrates the implementation of pulse compression using a matched transversal filter.
An array of waveforms has been used for pulse compression, including binary coding of the phase of a carrier signal (biphase modulation using binary codes). Perhaps the best-known codes for use in biphase modulation implementations of pulse compression are the Barker Codes. Other binary waveforms that have been used for pulse compression include pseudo-random codes and random binary codes. Nonbinary waveforms that have been used for pulse compression include FM modulated signals and polyphase codes.
A problem that has limited the utility of pulse compression and correlation receivers in radar systems has been the existence of temporal/range sidelobes in the correlation function of the radar waveform. These sidelobes allow out-of-range-gate returns, such as clutter, to compete with a target in a particular range gate. A number of research efforts have addressed this problem in the past, and several waveform designs have resulted in the potential reduction or elimination of the range sidelobe problem. For example, Barker codes (also known as perfect binary words) limit the range sidelobes. FIG. 3 illustrates the correlation function of a length 13 Barker Code. Barker codes are known for lengths only up to N=13, and they do not match the desired "perfect" range correlation property.
Application of Golay code pairs (also known as complementary sequences) involves processing two coded pulses at a time in a radar processor to eliminate the range sidelobes. These codes have the property that when their individual range sidelobes are combined (algebraic addition), the composite sidelobes completely cancel, yielding the desired perfect correlation property. Complementary sequences are known to exist for a limited number of sequence lengths, including N=2, 4, 8, 10, 16, 20, 32 and 40.
Several properties of binary code waveforms are desirable if they are to be used in implementing pulse compression in the target sensor component of a missile or fire-control system. These include very low or zero temporal sidelobes in the autocorrelation function and very low or zero cross-correlation with other binary codes that may be implemented in sensors deployed nearby. These properties would ensure that there would be little or no degradation in sensor system performance due to out-of-range clutter returns, multiple target sidelobes, or from mutual interference between deployed sensors using different codes.
Long binary codes with the desired properties are required in order to implement waveforms with large time-bandwidth products and large pulse-width compression ratios. This invention disclosure describes the structure and properties of such a waveform, called group-complementary codes.